Results
The results from the flow cytometry are given in Table 2, together with
the inferred ploidy levels, showing that the Sorbus hybrids
(Hedlundia or Aria ) are either triploids or tetraploids.
Size differences in length of the equatorial axis (E) of the measured
pollen are shown as boxplot in Figure 2. The mean values vary from 12 –
19.5 µm. The longest E value was measured in A. obtusifolia and
shortest equatorial axis is found in H. neglecta . S.
aucuparia and A. obtusifolia pollen show the least and most
variation, respectively (Figure 2). Comparing the length of E and
polyploidy level of the specimens there is a clear correlation between
the diploid, triploid, and tetraploid specimens, and length of the
equatorial axis (Figure 2).
Measurements of the polar axis (P) are given as boxplots in Figure 3 and
show that it is H. neglecta that has the shortest polar axis, andA. edulis , A. obtusifolia and H. hybrida 1 have the
longest. The measured means of all specimens vary between 13 and 26 µm.
The polar length in H. hybrida 1 grain shows most variation
within the measurements. Many specimens have a P between 16 and 20 µm.
When comparing the length of P and polyploidy level of the specimens,
there is no correlation between the diploid, triploid, and tetraploid
specimens, and length of their equatorial or polar axes. But, except forH. hybrida 1, Aria has larger pollen than the other
samples tested.
According to Erdtman (1952) the pollen shape can be categorized based on
the P/E ratio, and her separates pollen as oblate-spheroidal if the
ratio is 0.89–0.99, spheroidal when P/E is 1.00, prolate-spheroidal
between 1.01 and 1.14, sub-prolate between 1.15 and 1.33, and prolate if
the ratio is higher than 1.34 (Figure 4). The pollen measured in this
study all have a mean ratio > 1.00. Following this we see
that H. subsimilis , H. meinichii 1+2, H. subpinnata1+2, H. mougeotii , H. neglecta , A. rupicola , andH. subarranensis have a mean ratio between 1.01 and 1.14 and are
prolate-spheroidal. H. sognensis , H. lancifolia , H.
hybrida 2, and A. obtusifolia have a mean ratio in the
sub-prolate group, and H. hybrida 1, S. aucuparia andA. edulis are prolate.
In Figure 4, the ratios are compared to the polyploidy level of the
specimens. The two diploid species, A. edulis and S.
aucuparia have the largest P/E ratio in this study. The tetraploidH. hybrida 1 has significantly larger pollen compared to the
other triploid and tetraploid specimens.
Details of the surface of the pollen grains are not always clear while
observing through light microscope (LM), and for that reason, scanning
electron microscopy (SEM) was used to observe the surface structures and
details of the specimens. Selected images of all our studied specimens
are given in Figure 5, showing the polar view, the equatorial view, and
details of the surface pattern. The morphological characters of the
surface and the corresponding pollen shape for our study specimens are
listed in Table 3. Most visible is the long colpi in all our studied
specimens, directions and length of striae, number of perforations, and
the shape of the pollen grain in polar view.
The PCA ordination for the pollen measures of P and E, and their ratio
is given in Figure 6, where 99.91 % of the variation is explained by
the two first axes. The diploid S. aucuparia and A. edulishave a clearly higher ratio, as also seen in Figure 4. The tetraploid
specimens have a higher value of both P and E, compared to the triploid
specimens, but overlapping.
The results from the PCA ordination analysis of all the pollen
characters are shown in Figure 7, where 58.68 % of the variation is
explained by the two first axes. The diploid species S. aucupariaand A. edulis , have higher values for polar length (P), as also
seen in Figure 6, in addition to pollen perforation. The specimens are
grouped by genus in this figure and shows that Aria has larger
values of the measured characters and are grouped separately fromHedlundia . Hedlundia on the other hand, has large
variation between the different specimens, with the extremes H.
hybrida 1, H. lancifolia , and H. neglecta .